We do not manufacture amplifiers ourselves. If required we offer an amplifier suitable for your application. For stationary applications we recommend amplifiers in class AB operation. Amplifiers in class D operation are more suitable for mobile applications due to their low weight.
The dodecahedron m|dod250C can be used for sound insulation measurements up to a weighted sound reduction index of approx. RW = 45 dB, e. g. for measurements in administration buildings and education buildings. For higher sound insulation, as typically occurs in residential construction, the sound power of m|dod250C at low frequencies is insufficient due to the low enclosure volume. For this purpose we recommend the dodecahedron m|dod 360A.
The dodecahedron m|dod250C is perfectly suitable for room acoustic measurements. An exception are measurements in very large damped rooms such as concert halls and opera houses with more than 500 seats and especially measurements in occupied condition. For these applications the dodecahedron m|dod 360A is more suitable.
The diameter of the impedance tube determines the upper operating frequency up to which propagation of exclusively plane waves can be assumed. The smaller the inner diameter of the impedance tube is chosen, the higher the upper operating frequency is. But, a smaller impedance tube diameter requires a smaller specimen diameter, making it more difficult to achieve an airtight sealing of the specimen edge against the impedance tube wall.
The impedance tube has been developed for vertical mounting on a wall. The transmission of structure-borne sound from the wall to the impedance tube is considerably reduced by means of an elastic mounting. The vertical mounting simplifies specimen installation of flexible materials such as fabrics and nonwovens, since hereby the specimen is subject to the same weight force over the entire cross-section.
The determination of the transmissionrequires prior knowledge of the material to be examined, in order to decide whether a measurement will provide practice-oriented results. Well suited materials for transmission measurements are porous materials such as foams, composite fibers, etc. However, for stiff materials, the small sample size may cause the first eigenmode of the sample to be above the frequency range to be examined. In this case, the coupling of the sample to the tube wall may dominate the result.
For one measurement, at least two microphone positions must be recorded.
The third microphone position is used to extend the frequency range to low frequencies, as the greater distance between the microphones improves measurement accuracy at long wavelengths.
m|abstube exclusively supports the single-microphone method, where about 80 seconds are required for the entire measurement procedure. The multi-microphone method requires calibration of the microphones, has lower measurement accuracy and the time saving is insignificant to the total effort per measurement.
In rectangular impedance tubes, compensation of the first transverse modes can be performed with several microphones symmetrically distributed over the tube cross-section in order to increase the upper operating frequency. The signals of the microphones selected with respect to similar phases are combined in the software for evaluation, but can be analyzed individually, too.
No, the compressed air supply is not supplied by us. For the operation of the measurement system a dry and oil-free compressed air supply with an overpressure of at least 1 bar and a maximum of 15 bar with a connection via quick coupling NW 7.2 is necessary.
With a specimen cross-section corresponding to a diameter of 100 mm and a flow velocity of 0.5 mm/s, a specific airflow resistance of 40 Pa s/m can still be measured. If higher flow velocity and thus also higher pressure drops are permitted, the lower measurement limit shifts to 10 Pa s/m.
Firstly, a compressed air supply must be guaranteed (see also question 1). Secondly, a location should be selected where pressure fluctuations in air are as small as possible. The measurement method involves measuring the pressure difference between the pressure upstream of the specimen and the atmospheric pressure. For samples with very low airflow resistances, very small pressure differences of well below 1.0 Pa must be recorded, which will lead to unstable measurement results if the atmospheric pressure is unsteady. With high airflow resistances, measurements are made with very low volume flows. In this context, external air movements should be kept as low as possible.
The upper limit of measurement for airflow resistance is 20,000 Pa s/m. Practically, materials for acoustic applications with specific airflow resistances of more than 10,000 Pa s/m are “acoustically airtight”.
Sound absorption in the reverberation room m|abshall
Especially for small reverberation rooms, the possibility of using frequency-dependent and sample size-dependent correction values for different rooms is integrated in the software. The sets of correction values can also be changed later if, for example, a post-evaluation becomes necessary due to new reference values derived from round robin tests.
The equivalent sound absorption area or the sound absorption coefficient in one-third octave bands can be evaluated. Depending on the requirements and application area, an evaluation of derived criteria such as the practical sound absorption coefficients and the weighted sound absorption coefficient according to ISO 11654, as well as the NRC and the SAA according to ASTM C423 can also be performed.
The m|multimic can also be used with fewer microphones. Averaging over six independent microphone positions in the head area has proven to be the best compromise between required measurement uncertainty and effort.
With the artificial head, aurally appropriate directional recordings can be made at a specific position. The microphone array m|multimic enables the acquisition of a representative and reproducible mean value for the entire head area, even at high frequencies.
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